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Gel electrode imaging of deformation in aluminum alloys
Scripta METALLURGICA
Vol. 17, pp. 1371-1375, 1983 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
GEL ELECTRODE IMAGING OF DEFORMATION IN ALD~INUM ALLOYS
W. J . B a x t e r T. R. McKinney Physics Department G e n e r a l Motors R e s e a r c h L a b o r a t o r i e s Warren, M i c h i g a n 48090-9055
(Received August
24, 1983)
Introduction I n a r e c e n t s e r i e s o f r e p o r t s ( 1 - 4 ) , a new t e c h n i q u e was d e s c r i b e d which d e t e c t s and images f a t i g u e damage w i t h an e l e c t r o c h e m i c a l g e l e l e c t r o d e . F a t i g u e c r a c k s as s h o r t as 10 ~anwere imaged i n aluminum a l l o y s . I n a d d i t i o n , f a t i g u e d e f o r m a t i o n i n 1100-0 and 6061-T6 aluminum was d e t e c t e d and m e a s u r e d as e a r l y as 1Z o f t h e f a t i g u e l i f e . S i n c e t h i s d e f o r m a t i o n c o u l d n o t be d e t e c t e d by o p t i c a l o r s c a n n i n g e l e c t r o n m i c r o s c o p y , i t was n o t p o s s i b l e t o v e r i f y d i r e c t l y , by i n d e p e n d e n t o b s e r v a t i o n s , t h e n a t u r e o f t h e s u r f a c e c h a n g e s r e c o r d e d by t h e g e l e l e c t r o d e . The e x p l a n a t i o n o f f e r e d was t h a t t h e g e l e l e c t r o d e d e t e c t e d t h e p r e s e n c e o f f a t i g u e - i n d u c e d m i c r o c r a c k s i n t h e surface oxide film. T h i s p a p e r p r e s e n t s e v i d e n c e which c o n f i r m s t h i s h y p o t h e s i s . The e v i d e n c e i s p r o v i d e d by a s e r i e s o f e x p e r i m e n t s i n which g e l e l e c t r o d e images a r e compared w i t h p r i o r o b s e r v a tions of the specimens in a special photoemission electron microscope. ( T h i s i n s t r u m e n t , as d e s c r i b e d p r e v i o u s l y ( 5 ) , p r o v i d e s d i s t i n c t images o f m i c r o c r a c k s i n t h i n s u r f a c e o x i d e f i l m s on m e t a l s , a c a p a b i l i t y n o t s h a r e d by o t h e r forms o f m i c r o s c o p y . ) Experimental The e x p e r i m e n t a l p r o c e d u r e c o n s i s t e d o f t h r e e s t a g e s : (i) specimen preparation, which i n c l u d e d t h e g r o w t h o f a s u r f a c e o x i d e f i l m , ( i l ) d e f o r m a t i o n o f t h e specimen i n t h e p h o t o e m i s s i o n m i c r o s c o p e ( P ~ 4 ) , and ( i i i ) a f t e r removal o f t h e specimen from t h e PEN, g e l e l e c t r o d e imaging of t h e deformed r e g i o n . Specimens o f 1100-0, 6061-T6 and 7029 aluminum w e r e m a c h i n e d from s h e e t m a t e r i a l 1.5 mm t h i c k and t h e s u r f a c e t o be s t u d i e d was m e c h a n i c a l l y p o l i s h e d . (A r e q u i r e m e n t f o r good image q u a l i t y i n t h e PEN, b u t n o t n e c e s s a r y f o r g e l e l e c t r o d e i m a g i n g . ) The s p e c i m e n s were t h e n c l e a n e d by immersion i n c h r o m i c a c i d a t 70°C f o r f i v e m i n u t e s . N e x t , a s u r f a c e o x i d e f i l m was grown by a n o d i z a t i o n i n t a r t a r i c a c i d , t h e o x i d e t h i c k n e s s b e i n g c o n t r o l l e d by t h e a p p l i e d v o l t a g e ( t h i c k n e s s - 1.4 nm/V). F i n a l l y , a few l i n e s were g e n t l y s c r i b e d on t h e specimen w i t h a Bergsmen m i c r o h a r d n e s s t e s t e r . This removed t h e o x i d e t o expose a f r e s h m e t a l s u r f a c e a l o n g t h e s c r a t c h e s , so t h a t t h e y would a p p e a r as g u i d e m a r k e r s i n b o t h t h e p h o t o e l e c t r o n and g e l e l e c t r o d e images. The s p e c i m e n s were deformed by b e n d i n g i n t h e s p e c i m e n chamber o f t h e PEN. Some were b e n t unidirectionally t o p r o d u c e t e n s i l e d e f o r m a t i o n o f t h e s u r f a c e , w h i l e o t h e r s were f a t i g u e d by b e n d i n g a t low c y c l i c s u r f a c e s t r a i n s . I n t h e p h o t o e m i s s i o n m i c r o s c o p e , e l e c t r o n s e m i t t e d from t h e s p e c i m e n form a m a g n i f i e d image o f t h e s u r f a c e on a f l u o r e s c e n t s c r e e n ( 5 ) . As t h e specimen i s d e f o r m e d , t h e d e v e l o p m e n t o f m i c r o c r a c k s i n t h e s u r f a c e o x i d e f i l m i s i d e n t i f i e d by t h e a p p e a r a n c e o f r e g i o n s of more i n t e n s e e l e c t r o n e m i s s i o n ( e x o e l e c t r o n s ) . T h e s e a p p e a r as w h i t e l i n e s on t h e m i c r o g r a p h s shown i n t h e n e x t s e c t i o n . As d e s c r i b e d p r e v i o u s l y ( 1 - 4 ) , t h e imaging g e l e l e c t r o d e was formed as f o l l o w s . An a g a r g e l was p r e p a r e d c o n t a i n i n g 0 . 2 m o l a r p o t a s s i u m i o d i d e , 0.05 m o l a r b o r a x and 0 . 2 m o l a r s t a r c h . This warm f l u i d m i x t u r e was d i s p e n s e d i n t o s h o r t l e n g t h s (~3 cm) o f p l a s t i c t u b e 6 mm i n d i a m e t e r . The t u b e was o v e r f i l l e d so t h a t , upon c o o l i n g , t h e l i q u i d formed a smooth h e m i s p h e r i c a l g e l t i p a t one
1371 0036-9748/83 $3.00 + .00 Copyright (c) 1983 Pergamon Press Ltd.
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end of the tube. A piece of aluminum wire was sealed into the other end of the tube to serve as a cathode. After approximately five minutes a flexible skin formed on the gel tip. To form an image in this skin, the gel tip was pressed against the surface of the specimen and a pulse of negative p o t e n t i a l was a p p l i e d t o t h e c a t h o d e . The v o l t a g e and d u r a t i o n o f t h e p u l s e was s e l e c t e d t o produce images w i t h good s p a t i a l r e s o l u t i o n t o f a c i l i t a t e detailed correlation with the photoelectron micrographs. The g e l images were viewed and p h o t o g r a p h e d w i t h an o p t i c a l m i c r o s c o p e . Results In viewing the following comparisons of photoelectron and gel electrode images, it should be noted that the condition of the microcracks in the oxide film differs slightly during observations by the two techniques. The PEM images the microcracks by virtue of electrons emitted from the metal surface freshly exposed at the base of the microcrack. During subsequent exposure to air at atmospheric pressure this metal surface oxidizes very quickly, and a thin (~4 um) oxide film is formed at the base of the microcrack. This thin oxide layer suppresses the photoelectron emission so that further imaging in the PEN of the microcracks would be very dlfficult. On the other hand, during gel electrode imaging this thin oxide layer is present and yet the technique causes sufficient current flow to form an image of the mlcrocracks. (I) 100 um Oxide on 1100 Aluminum During tensile deformation of aluminum coated with a thick oxide film, straight and welldefined microcracks are produced in the surface oxide (6,7). This distinctive mode of oxide r u p t u r e p r o v i d e s an i n t e r e s t i n g v e h i c l e f o r c o m p a r i n g t h e imaging c a p a b i l i t l e s o f t h e PEM and t h e gel electrode. The p h o t o e l e c t r o n m i c r o g r a p h i n F i g . l ( a ) shows t h e s u r f a c e o f a specimen a f t e r a t e n s i l e s t r a i n o f 4 x I0 ~. The two b r o a d l i n e s p a r a l l e l t o t h e s t r e s s d i r e c t i o n a r e f i d u c i a r y scratches. The a r r a y o f f i n e r l i n e s o f p h o t o e l e c t r o n e m i s s i o n i d e n t i f y t h e p r e s e n c e o f m i c r o c r a c k s aligned perpendicularly to the stress direction. The o p t i c a l m i c r o g r a p h i n F i g . l ( b ) shows a p o r t i o n o f a g e l e l e c t r o d e image formed by f o u r p u l s e s o f 400 v o l t s , each o f 30 ~sec d u r a t i o n . T h i s image c o r r e s p o n d s t o t h e same a r e a of t h e s p e c i m e n as shown i n t h e p h o t o e l e c t r o n image i n F i g . l ( a ) . (Note t h a t t h e f i e l d o f view o f t h e g e l e l e c t r o d e image i s always l a r g e r t h a n t h a t o f t h e PEN.) The f e a t u r e s i n t h e s e two images c o r r e l a t e v e r y p r e c i s e l y , c l e a r l y d e m o n s t r a t i n g t h a t t h e g e l e l e c t r o d e has imaged the microcracks in the oxide film. (2) 28 um Oxide on 7029 Aluminum 7029 aluminum is a high strength alloy, wherein the interior of the grains are strengthened by a high density of very small precipitates. But a d j a c e n t t o t h e g r a i n b o u n d a r i e s t h e r e i s a v e r y t h i n r e g i o n o f s o f t e r m a t e r i a l , o f t e n r e f e r r e d t o as t h e p r e c i p i t a t e f r e e zone (PFZ). An i n t e r e s t i n g c o n s e q u e n c e o f t h i s m i c r o s t r u c t u r e i s t h a t p l a s t i c d e f o r m a t i o n t e n d s t o be c o n f i n e d w i t h i n t h e PFZ. Thus d u r i n g t e n s i l e d e f o r m a t i o n t h e s u r f a c e o x i d e f i l m d e v e l o p s m i c r o c r a c k s p r i m a r i l y along the grain boundaries. This results in a very distinctive g e o m e t r i c a l p a t t e r n which a g a i n p r o v i d e s an i n t e r e s t i n g b a s i s f o r c o m p a r i n g p h o t o e l e c t r o n and g e l e l e c t r o d e i m a g e s .
~he o p t i c a l m i c r o g r a p h i n F i g . 2 ( a ) shows t h e s u r f a c e o f a specimen a f t e r a t e n s i l e s t r a i n o f 4 x 1 0 - - ; two f i d u c i a l s c r a t c h e s and t h e g r a i n s t r u c t u r e a r e c l e a r l y v i s i b l e , b u t t h e r e i s v i r t u a l l y no e v i d e n c e o f t h e d e f o r m a t i o n . The p h o t o e l e c t r o n m i c r o g r a p h i n F i g . 2 ( b ) c o r r e s p o n d s t o o n l y t h e central region of Fig. 2(a); the lines of exoelectron emission delineate the microcracks in the o x i d e f i l m a l o n g some, b u t n o t a l l , o f t h e g r a i n b o u n d a r i e s . A portion of the corresponding gel e l e c t r o d e image i s shown i n t h e o p t i c a l m i c r o g r a p h i n F i g . 2 ( c ) . T h i s image was formed by a 10 V, 100 ms p u l s e . The g e l e l e c t r o d e h a s imaged a l a r g e number o f t h e g r a i n b o u n d a r i e s , i n c l u d i n g a l l t h o s e d i s p l a y i n g e x o e l e c t r o n e m i s s i o n i n t h e p h o t o e l e c t r o n image. However, n o t a l l t h e g r a i n b o u n d a r i e s a r e imaged, b u t r a t h e r p r i m a r i l y t h o s e which a r e a t an a n g l e t o t h e s t r e s s d i r e c t i o n . S i n c e we found t h a t no g r a i n b o u n d a r i e s c o u l d be imaged i n a n o n - d e f o r m e d s p e c i m e n , we a g a i n c o n c l u d e t h a t t h e g e l e l e c t r o d e h a s imaged o n l y t h e g r a i n b o u n d a r i e s where m l c r o c r a c k s h a v e been produced in the oxide film. (37 14 um Oxide on 1100 Aluminum These e x p e r i m e n t s were p e r f o r m e d t o s i m u l a t e t h e c o n d i t i o n s o f t h e e a r l i e r m e a s u r e m e n t s of f a t i g u e d e f o r m a t i o n i n 1100 aluminum (27. However, t h e y d i f f e r e d i n one r e s p e c t , namely t h e selection of voltage pulse during gel electrode imaging. I n t h e p r e v i o u s s t u d y (27, a 5 v o l t , 5 s e c o n d p u l s e was a p p l i e d t o maximize t h e s e n s i t i v i t y . T h i s r e s u l t s i n h i g h l y o v e r e x p o s e d images c o n s i s t i n g o f a r r a y s o f c i r c u l a r s p o t s , which mask any g e o m e t r i c a l a s p e c t s o f t h e c o r r e s p o n d i n g
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fatigue-induced microcracks. Such images were found to be extremely difficult to correlate with photoelectron micrographs. Therefore in these experiments the gel electrode images were formed by a 50 volt, I mi11isecond pulse to obtain improved spatial resolution. The images shown in Fig. 3 were obtained from a specimen after 7 x 104 cycles. As shown by the many sources of exoelectron emission in Fig. 3(a), the rupturing of the oxide film is quite extensive. This is also apparent from the corresponding gel electrode image shown in Fig. 3(b). The overall correlation between the two images in Fig. 3 is quite good, although it is obscured to some extent by the poorer resolution in the gel electrode image, where the fine arrays of microcracks are simply imaged as single, much broader bands. (4) 14 nm Oxide on 6061-T6 Aluminum These experiments were performed to simulate the conditions of the earlier measurements of fatigue deformation in 6061-T6 aluminum (4). Again it was found that a 5 volt, 5 second pulse overexposed the gel electrode image, whereas 50 V, I ms pulses produced images with sufficient spatial resolution to permit comparisons with photoelectron micrographs. The photoelectron micrograph in Fig. 4(a) shows exoelectron emission from arrays of slip bands produced after 104 fatigue cycles at a tensile strain of 6.8 x i0 -~. An enlarged view of the corresponding gel electrode image is shown in Fig. 4(b). This image, which was formed by two 50 V, I ms pulses, is slightly overexposed: for example the cluster of slip bands at 'A' are not resolved. Nevertheless, all the sites of oxide rupture identified by the PEM image are detected by the gel electrode image, with the possible exception of the very fine lines (B) at the tip of the fiducial scratch. In this experiment it was noted that during subsequent gel electrode imaging, under identical conditions, less current was passed and the image was much weaker but with better spatial resolution. This is illustrated in Fig. 4(c) where the individual slip bands at regions such as A are clearly resolved. The bands at B are still barely discernible, but imaging in this region could be hampered by the close proximity of the fiducial scratch. The limit of spatial resolution in the gel electrode image appears to be about I0 tm. Summary The above series of comparisons of gel electrode and photoemission electron images demonstrate that the gel electrode technique can detect and image deformation-induced microcracks in surface anodic oxide films on aluminum. Although the spatial resolution of the gel electrode image is not as good as that of the PEM, it is considerably improved by increasing the magnitude of the voltage pulse and decreasing its duration. In these experiments gel electrode images were obtained with a spatial resolution of I0 tan. This was sufficient to clearly define the mode of fracture of the oxide film. For example, after tensile deformation of 1100-0 aluminum the gel electrode clearly imaged the microcracks in a I00 nm oxide which are produced perpendicular to the stress axis, while in the case of deformed 7029 aluminum, the gel electrode image revealed the rupture of a 28 nm oxide along the grain boundaries. On the other hand, during the early stages of fatigue of 6061-T6 and 1100-0 aluminum coated with a 14 nm oxide film, the gel electrode imaged microcracks produced by slip band deformation. The sensitivity of the gel electrode to the presence of microcracks in anodic surface oxide films approximates that of the PEM, despite the fundamentally different imaging process. The PEM r e q u i r e s t h e c r e a t i o n ( i n vacuo) o f f r e s h m e t a l s u r f a c e s a t the b a s e o f t h e m i c r o c r a c k s , whereas the g e l e l e c t r o d e can image t h e m i c r o c r a c k s even a f t e r t h o s e m e t a l s u r f a c e s have been r e o x i d i z e d by exposure to the atmosphere. References i. 2. 3. 4. 5. 6. 7.
B a x t e r , Her. T r a n s . 13A, 1413 (1982). N . J . B a x t e r , N e t . T r a n s . 1-~A, 1421 (1982). N . J . B a x t e r , I n t . J . F a t i g u e , p. 37 ( J a n u a r y 1983). W . J . B a x t e r , "Oxide F i l m s : Q u a n t i t a t i v e S e n s o r s o f Metal F a t i g u e , " G e n e r a l N o t o r s R e s e a r c h P u b l i c a t i o n GHR-3958, 28 J a n u a r y 1982. (To a p p e a r i n P r o c e e d i n g s o f ASTH C o n f e r e n c e on ' Q u a n t i t a t i v e Heasurement o f N e t a l F a t i g u e ' , D e a r b o r n , N i c h i g a n , Hay 1982.) N . J . B a x t e r and S. R. Rouze, J . Appl. P h y s . , 44, 4400 (1973). C. E d e l e a n u and T. J . Law, P h i l . Hag. 7, 573 (1962). J . C . G r o s s k r e u t z , J . B l e c t r o c h e m . S o c : 116, 1132 ( 1 9 6 9 ) . N.J.
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PIG. 1 (a) P h o t o e l e c t r o n micrograph o f specimen o f 1100 alum_~num coated w i t h 100 ~ oxide a f t e r t e n s i l e d e f o r m a t i o n o f 4 x 10 . E x o e l e c t r o n s d e f i n e m i c r o c r a c k s i n t h e o x i d e and t h e two f i d u c i a l s c r a t c h e s . ( b ) O p t i c a l m i c r o g r a p h o f g e l e l e c t r o d e image formed by f o u r p u l s e s o f 400 v o l t s and 30 ~sec d u r a t i o n .
ii i iiiii!!iiliii i !!i!i~!!(a!i!i!!!i !i! i !!ii!¸~¸i!!!iiii ~Ii !
! ib~i i!/ii!C i¸i! ' i
(ct
FIG. 2 ( a ) O p t i c a l m i c r o s r a p h o f s p e c i m e n o f 7029 aluminum showinE E r a i n s t r u c t u r e and f i d u c i a l s c r a t c h e s . ( b ) P h o t o e l e c t r o n i~age s h o v i n g e x o e l e c t r o n e m i s s i o n from mic_r~cracks i n t h e 28 n n o x i d e f i l m a f t e r t e n s i l e d e f o r m a t i o n o f 4 x 10 . ( c ) O p t i c a l n l c r o g r a p h o f g e l e l e c t r o d e image formed by e 10 V, 100 ms pulse. Note:
(e) is a airror
image o f ( b ) and ( c ) .
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FIG. 3 (a) Photoelectron micrograph ~f specimen of 1100 aluminum, coated with a 14 nm oxide, after 7 x 10 cycles. (b) Gel electrode image of same region formed by a 50 V I ms pulse.
~J
(a|
(b~
(¢)
FIG. 4 (a) Photoelectron micrograph of 6~61-T6 aluminum after 104 fatigue c y c l e s a t a s t r a i n o f 6 . 8 x 10 . Oxide t h i c k n e s s 14 nm. (b) Gel e l e c t r o d e image o f same r e g i o n formed by two 50 V, 1 ms p u l s e s . ( c ) S u b s e q u e n t r e p e a t o f g e l e l e c t r o d e imaEe formed by two 50 V, 1 I s pulses.
Vol. 17, pp. 1371-1375, 1983 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
GEL ELECTRODE IMAGING OF DEFORMATION IN ALD~INUM ALLOYS
W. J . B a x t e r T. R. McKinney Physics Department G e n e r a l Motors R e s e a r c h L a b o r a t o r i e s Warren, M i c h i g a n 48090-9055
(Received August
24, 1983)
Introduction I n a r e c e n t s e r i e s o f r e p o r t s ( 1 - 4 ) , a new t e c h n i q u e was d e s c r i b e d which d e t e c t s and images f a t i g u e damage w i t h an e l e c t r o c h e m i c a l g e l e l e c t r o d e . F a t i g u e c r a c k s as s h o r t as 10 ~anwere imaged i n aluminum a l l o y s . I n a d d i t i o n , f a t i g u e d e f o r m a t i o n i n 1100-0 and 6061-T6 aluminum was d e t e c t e d and m e a s u r e d as e a r l y as 1Z o f t h e f a t i g u e l i f e . S i n c e t h i s d e f o r m a t i o n c o u l d n o t be d e t e c t e d by o p t i c a l o r s c a n n i n g e l e c t r o n m i c r o s c o p y , i t was n o t p o s s i b l e t o v e r i f y d i r e c t l y , by i n d e p e n d e n t o b s e r v a t i o n s , t h e n a t u r e o f t h e s u r f a c e c h a n g e s r e c o r d e d by t h e g e l e l e c t r o d e . The e x p l a n a t i o n o f f e r e d was t h a t t h e g e l e l e c t r o d e d e t e c t e d t h e p r e s e n c e o f f a t i g u e - i n d u c e d m i c r o c r a c k s i n t h e surface oxide film. T h i s p a p e r p r e s e n t s e v i d e n c e which c o n f i r m s t h i s h y p o t h e s i s . The e v i d e n c e i s p r o v i d e d by a s e r i e s o f e x p e r i m e n t s i n which g e l e l e c t r o d e images a r e compared w i t h p r i o r o b s e r v a tions of the specimens in a special photoemission electron microscope. ( T h i s i n s t r u m e n t , as d e s c r i b e d p r e v i o u s l y ( 5 ) , p r o v i d e s d i s t i n c t images o f m i c r o c r a c k s i n t h i n s u r f a c e o x i d e f i l m s on m e t a l s , a c a p a b i l i t y n o t s h a r e d by o t h e r forms o f m i c r o s c o p y . ) Experimental The e x p e r i m e n t a l p r o c e d u r e c o n s i s t e d o f t h r e e s t a g e s : (i) specimen preparation, which i n c l u d e d t h e g r o w t h o f a s u r f a c e o x i d e f i l m , ( i l ) d e f o r m a t i o n o f t h e specimen i n t h e p h o t o e m i s s i o n m i c r o s c o p e ( P ~ 4 ) , and ( i i i ) a f t e r removal o f t h e specimen from t h e PEN, g e l e l e c t r o d e imaging of t h e deformed r e g i o n . Specimens o f 1100-0, 6061-T6 and 7029 aluminum w e r e m a c h i n e d from s h e e t m a t e r i a l 1.5 mm t h i c k and t h e s u r f a c e t o be s t u d i e d was m e c h a n i c a l l y p o l i s h e d . (A r e q u i r e m e n t f o r good image q u a l i t y i n t h e PEN, b u t n o t n e c e s s a r y f o r g e l e l e c t r o d e i m a g i n g . ) The s p e c i m e n s were t h e n c l e a n e d by immersion i n c h r o m i c a c i d a t 70°C f o r f i v e m i n u t e s . N e x t , a s u r f a c e o x i d e f i l m was grown by a n o d i z a t i o n i n t a r t a r i c a c i d , t h e o x i d e t h i c k n e s s b e i n g c o n t r o l l e d by t h e a p p l i e d v o l t a g e ( t h i c k n e s s - 1.4 nm/V). F i n a l l y , a few l i n e s were g e n t l y s c r i b e d on t h e specimen w i t h a Bergsmen m i c r o h a r d n e s s t e s t e r . This removed t h e o x i d e t o expose a f r e s h m e t a l s u r f a c e a l o n g t h e s c r a t c h e s , so t h a t t h e y would a p p e a r as g u i d e m a r k e r s i n b o t h t h e p h o t o e l e c t r o n and g e l e l e c t r o d e images. The s p e c i m e n s were deformed by b e n d i n g i n t h e s p e c i m e n chamber o f t h e PEN. Some were b e n t unidirectionally t o p r o d u c e t e n s i l e d e f o r m a t i o n o f t h e s u r f a c e , w h i l e o t h e r s were f a t i g u e d by b e n d i n g a t low c y c l i c s u r f a c e s t r a i n s . I n t h e p h o t o e m i s s i o n m i c r o s c o p e , e l e c t r o n s e m i t t e d from t h e s p e c i m e n form a m a g n i f i e d image o f t h e s u r f a c e on a f l u o r e s c e n t s c r e e n ( 5 ) . As t h e specimen i s d e f o r m e d , t h e d e v e l o p m e n t o f m i c r o c r a c k s i n t h e s u r f a c e o x i d e f i l m i s i d e n t i f i e d by t h e a p p e a r a n c e o f r e g i o n s of more i n t e n s e e l e c t r o n e m i s s i o n ( e x o e l e c t r o n s ) . T h e s e a p p e a r as w h i t e l i n e s on t h e m i c r o g r a p h s shown i n t h e n e x t s e c t i o n . As d e s c r i b e d p r e v i o u s l y ( 1 - 4 ) , t h e imaging g e l e l e c t r o d e was formed as f o l l o w s . An a g a r g e l was p r e p a r e d c o n t a i n i n g 0 . 2 m o l a r p o t a s s i u m i o d i d e , 0.05 m o l a r b o r a x and 0 . 2 m o l a r s t a r c h . This warm f l u i d m i x t u r e was d i s p e n s e d i n t o s h o r t l e n g t h s (~3 cm) o f p l a s t i c t u b e 6 mm i n d i a m e t e r . The t u b e was o v e r f i l l e d so t h a t , upon c o o l i n g , t h e l i q u i d formed a smooth h e m i s p h e r i c a l g e l t i p a t one
1371 0036-9748/83 $3.00 + .00 Copyright (c) 1983 Pergamon Press Ltd.
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end of the tube. A piece of aluminum wire was sealed into the other end of the tube to serve as a cathode. After approximately five minutes a flexible skin formed on the gel tip. To form an image in this skin, the gel tip was pressed against the surface of the specimen and a pulse of negative p o t e n t i a l was a p p l i e d t o t h e c a t h o d e . The v o l t a g e and d u r a t i o n o f t h e p u l s e was s e l e c t e d t o produce images w i t h good s p a t i a l r e s o l u t i o n t o f a c i l i t a t e detailed correlation with the photoelectron micrographs. The g e l images were viewed and p h o t o g r a p h e d w i t h an o p t i c a l m i c r o s c o p e . Results In viewing the following comparisons of photoelectron and gel electrode images, it should be noted that the condition of the microcracks in the oxide film differs slightly during observations by the two techniques. The PEM images the microcracks by virtue of electrons emitted from the metal surface freshly exposed at the base of the microcrack. During subsequent exposure to air at atmospheric pressure this metal surface oxidizes very quickly, and a thin (~4 um) oxide film is formed at the base of the microcrack. This thin oxide layer suppresses the photoelectron emission so that further imaging in the PEN of the microcracks would be very dlfficult. On the other hand, during gel electrode imaging this thin oxide layer is present and yet the technique causes sufficient current flow to form an image of the mlcrocracks. (I) 100 um Oxide on 1100 Aluminum During tensile deformation of aluminum coated with a thick oxide film, straight and welldefined microcracks are produced in the surface oxide (6,7). This distinctive mode of oxide r u p t u r e p r o v i d e s an i n t e r e s t i n g v e h i c l e f o r c o m p a r i n g t h e imaging c a p a b i l i t l e s o f t h e PEM and t h e gel electrode. The p h o t o e l e c t r o n m i c r o g r a p h i n F i g . l ( a ) shows t h e s u r f a c e o f a specimen a f t e r a t e n s i l e s t r a i n o f 4 x I0 ~. The two b r o a d l i n e s p a r a l l e l t o t h e s t r e s s d i r e c t i o n a r e f i d u c i a r y scratches. The a r r a y o f f i n e r l i n e s o f p h o t o e l e c t r o n e m i s s i o n i d e n t i f y t h e p r e s e n c e o f m i c r o c r a c k s aligned perpendicularly to the stress direction. The o p t i c a l m i c r o g r a p h i n F i g . l ( b ) shows a p o r t i o n o f a g e l e l e c t r o d e image formed by f o u r p u l s e s o f 400 v o l t s , each o f 30 ~sec d u r a t i o n . T h i s image c o r r e s p o n d s t o t h e same a r e a of t h e s p e c i m e n as shown i n t h e p h o t o e l e c t r o n image i n F i g . l ( a ) . (Note t h a t t h e f i e l d o f view o f t h e g e l e l e c t r o d e image i s always l a r g e r t h a n t h a t o f t h e PEN.) The f e a t u r e s i n t h e s e two images c o r r e l a t e v e r y p r e c i s e l y , c l e a r l y d e m o n s t r a t i n g t h a t t h e g e l e l e c t r o d e has imaged the microcracks in the oxide film. (2) 28 um Oxide on 7029 Aluminum 7029 aluminum is a high strength alloy, wherein the interior of the grains are strengthened by a high density of very small precipitates. But a d j a c e n t t o t h e g r a i n b o u n d a r i e s t h e r e i s a v e r y t h i n r e g i o n o f s o f t e r m a t e r i a l , o f t e n r e f e r r e d t o as t h e p r e c i p i t a t e f r e e zone (PFZ). An i n t e r e s t i n g c o n s e q u e n c e o f t h i s m i c r o s t r u c t u r e i s t h a t p l a s t i c d e f o r m a t i o n t e n d s t o be c o n f i n e d w i t h i n t h e PFZ. Thus d u r i n g t e n s i l e d e f o r m a t i o n t h e s u r f a c e o x i d e f i l m d e v e l o p s m i c r o c r a c k s p r i m a r i l y along the grain boundaries. This results in a very distinctive g e o m e t r i c a l p a t t e r n which a g a i n p r o v i d e s an i n t e r e s t i n g b a s i s f o r c o m p a r i n g p h o t o e l e c t r o n and g e l e l e c t r o d e i m a g e s .
~he o p t i c a l m i c r o g r a p h i n F i g . 2 ( a ) shows t h e s u r f a c e o f a specimen a f t e r a t e n s i l e s t r a i n o f 4 x 1 0 - - ; two f i d u c i a l s c r a t c h e s and t h e g r a i n s t r u c t u r e a r e c l e a r l y v i s i b l e , b u t t h e r e i s v i r t u a l l y no e v i d e n c e o f t h e d e f o r m a t i o n . The p h o t o e l e c t r o n m i c r o g r a p h i n F i g . 2 ( b ) c o r r e s p o n d s t o o n l y t h e central region of Fig. 2(a); the lines of exoelectron emission delineate the microcracks in the o x i d e f i l m a l o n g some, b u t n o t a l l , o f t h e g r a i n b o u n d a r i e s . A portion of the corresponding gel e l e c t r o d e image i s shown i n t h e o p t i c a l m i c r o g r a p h i n F i g . 2 ( c ) . T h i s image was formed by a 10 V, 100 ms p u l s e . The g e l e l e c t r o d e h a s imaged a l a r g e number o f t h e g r a i n b o u n d a r i e s , i n c l u d i n g a l l t h o s e d i s p l a y i n g e x o e l e c t r o n e m i s s i o n i n t h e p h o t o e l e c t r o n image. However, n o t a l l t h e g r a i n b o u n d a r i e s a r e imaged, b u t r a t h e r p r i m a r i l y t h o s e which a r e a t an a n g l e t o t h e s t r e s s d i r e c t i o n . S i n c e we found t h a t no g r a i n b o u n d a r i e s c o u l d be imaged i n a n o n - d e f o r m e d s p e c i m e n , we a g a i n c o n c l u d e t h a t t h e g e l e l e c t r o d e h a s imaged o n l y t h e g r a i n b o u n d a r i e s where m l c r o c r a c k s h a v e been produced in the oxide film. (37 14 um Oxide on 1100 Aluminum These e x p e r i m e n t s were p e r f o r m e d t o s i m u l a t e t h e c o n d i t i o n s o f t h e e a r l i e r m e a s u r e m e n t s of f a t i g u e d e f o r m a t i o n i n 1100 aluminum (27. However, t h e y d i f f e r e d i n one r e s p e c t , namely t h e selection of voltage pulse during gel electrode imaging. I n t h e p r e v i o u s s t u d y (27, a 5 v o l t , 5 s e c o n d p u l s e was a p p l i e d t o maximize t h e s e n s i t i v i t y . T h i s r e s u l t s i n h i g h l y o v e r e x p o s e d images c o n s i s t i n g o f a r r a y s o f c i r c u l a r s p o t s , which mask any g e o m e t r i c a l a s p e c t s o f t h e c o r r e s p o n d i n g
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fatigue-induced microcracks. Such images were found to be extremely difficult to correlate with photoelectron micrographs. Therefore in these experiments the gel electrode images were formed by a 50 volt, I mi11isecond pulse to obtain improved spatial resolution. The images shown in Fig. 3 were obtained from a specimen after 7 x 104 cycles. As shown by the many sources of exoelectron emission in Fig. 3(a), the rupturing of the oxide film is quite extensive. This is also apparent from the corresponding gel electrode image shown in Fig. 3(b). The overall correlation between the two images in Fig. 3 is quite good, although it is obscured to some extent by the poorer resolution in the gel electrode image, where the fine arrays of microcracks are simply imaged as single, much broader bands. (4) 14 nm Oxide on 6061-T6 Aluminum These experiments were performed to simulate the conditions of the earlier measurements of fatigue deformation in 6061-T6 aluminum (4). Again it was found that a 5 volt, 5 second pulse overexposed the gel electrode image, whereas 50 V, I ms pulses produced images with sufficient spatial resolution to permit comparisons with photoelectron micrographs. The photoelectron micrograph in Fig. 4(a) shows exoelectron emission from arrays of slip bands produced after 104 fatigue cycles at a tensile strain of 6.8 x i0 -~. An enlarged view of the corresponding gel electrode image is shown in Fig. 4(b). This image, which was formed by two 50 V, I ms pulses, is slightly overexposed: for example the cluster of slip bands at 'A' are not resolved. Nevertheless, all the sites of oxide rupture identified by the PEM image are detected by the gel electrode image, with the possible exception of the very fine lines (B) at the tip of the fiducial scratch. In this experiment it was noted that during subsequent gel electrode imaging, under identical conditions, less current was passed and the image was much weaker but with better spatial resolution. This is illustrated in Fig. 4(c) where the individual slip bands at regions such as A are clearly resolved. The bands at B are still barely discernible, but imaging in this region could be hampered by the close proximity of the fiducial scratch. The limit of spatial resolution in the gel electrode image appears to be about I0 tm. Summary The above series of comparisons of gel electrode and photoemission electron images demonstrate that the gel electrode technique can detect and image deformation-induced microcracks in surface anodic oxide films on aluminum. Although the spatial resolution of the gel electrode image is not as good as that of the PEM, it is considerably improved by increasing the magnitude of the voltage pulse and decreasing its duration. In these experiments gel electrode images were obtained with a spatial resolution of I0 tan. This was sufficient to clearly define the mode of fracture of the oxide film. For example, after tensile deformation of 1100-0 aluminum the gel electrode clearly imaged the microcracks in a I00 nm oxide which are produced perpendicular to the stress axis, while in the case of deformed 7029 aluminum, the gel electrode image revealed the rupture of a 28 nm oxide along the grain boundaries. On the other hand, during the early stages of fatigue of 6061-T6 and 1100-0 aluminum coated with a 14 nm oxide film, the gel electrode imaged microcracks produced by slip band deformation. The sensitivity of the gel electrode to the presence of microcracks in anodic surface oxide films approximates that of the PEM, despite the fundamentally different imaging process. The PEM r e q u i r e s t h e c r e a t i o n ( i n vacuo) o f f r e s h m e t a l s u r f a c e s a t the b a s e o f t h e m i c r o c r a c k s , whereas the g e l e l e c t r o d e can image t h e m i c r o c r a c k s even a f t e r t h o s e m e t a l s u r f a c e s have been r e o x i d i z e d by exposure to the atmosphere. References i. 2. 3. 4. 5. 6. 7.
B a x t e r , Her. T r a n s . 13A, 1413 (1982). N . J . B a x t e r , N e t . T r a n s . 1-~A, 1421 (1982). N . J . B a x t e r , I n t . J . F a t i g u e , p. 37 ( J a n u a r y 1983). W . J . B a x t e r , "Oxide F i l m s : Q u a n t i t a t i v e S e n s o r s o f Metal F a t i g u e , " G e n e r a l N o t o r s R e s e a r c h P u b l i c a t i o n GHR-3958, 28 J a n u a r y 1982. (To a p p e a r i n P r o c e e d i n g s o f ASTH C o n f e r e n c e on ' Q u a n t i t a t i v e Heasurement o f N e t a l F a t i g u e ' , D e a r b o r n , N i c h i g a n , Hay 1982.) N . J . B a x t e r and S. R. Rouze, J . Appl. P h y s . , 44, 4400 (1973). C. E d e l e a n u and T. J . Law, P h i l . Hag. 7, 573 (1962). J . C . G r o s s k r e u t z , J . B l e c t r o c h e m . S o c : 116, 1132 ( 1 9 6 9 ) . N.J.
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PIG. 1 (a) P h o t o e l e c t r o n micrograph o f specimen o f 1100 alum_~num coated w i t h 100 ~ oxide a f t e r t e n s i l e d e f o r m a t i o n o f 4 x 10 . E x o e l e c t r o n s d e f i n e m i c r o c r a c k s i n t h e o x i d e and t h e two f i d u c i a l s c r a t c h e s . ( b ) O p t i c a l m i c r o g r a p h o f g e l e l e c t r o d e image formed by f o u r p u l s e s o f 400 v o l t s and 30 ~sec d u r a t i o n .
ii i iiiii!!iiliii i !!i!i~!!(a!i!i!!!i !i! i !!ii!¸~¸i!!!iiii ~Ii !
! ib~i i!/ii!C i¸i! ' i
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FIG. 2 ( a ) O p t i c a l m i c r o s r a p h o f s p e c i m e n o f 7029 aluminum showinE E r a i n s t r u c t u r e and f i d u c i a l s c r a t c h e s . ( b ) P h o t o e l e c t r o n i~age s h o v i n g e x o e l e c t r o n e m i s s i o n from mic_r~cracks i n t h e 28 n n o x i d e f i l m a f t e r t e n s i l e d e f o r m a t i o n o f 4 x 10 . ( c ) O p t i c a l n l c r o g r a p h o f g e l e l e c t r o d e image formed by e 10 V, 100 ms pulse. Note:
(e) is a airror
image o f ( b ) and ( c ) .
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FIG. 3 (a) Photoelectron micrograph ~f specimen of 1100 aluminum, coated with a 14 nm oxide, after 7 x 10 cycles. (b) Gel electrode image of same region formed by a 50 V I ms pulse.
~J
(a|
(b~
(¢)
FIG. 4 (a) Photoelectron micrograph of 6~61-T6 aluminum after 104 fatigue c y c l e s a t a s t r a i n o f 6 . 8 x 10 . Oxide t h i c k n e s s 14 nm. (b) Gel e l e c t r o d e image o f same r e g i o n formed by two 50 V, 1 ms p u l s e s . ( c ) S u b s e q u e n t r e p e a t o f g e l e l e c t r o d e imaEe formed by two 50 V, 1 I s pulses.